Ceramic Metal Halide (“CMH”) lamps are special types of High intensity Discharge (“HID”) lamps, and more specifically relate to Metal Halide, arc discharge lamps. These lamps are known to operate at high pressures and at high temperatures, and to have discharge vessels (frequently referred to as “arc tubes”) made of a ceramic material. The arc tubes of CMH lamps include an ionizable fill of a noble gas such as Neon (Ne), Argon (Ar), Krypton (Kr) or Xenon (Xe) or a mixture of thereof, mercury or some of its alternatives the vapor of which serves as a buffer gas, and a mixture of metal halide salts such as, for example, NaI (sodium iodide), IlI (thallium iodide), CaI2 (calcium iodide) and REIn (where REIn refers to rare-earth iodides). This mixture of metal halide salts (sometimes referred to as a “metal halide dose”) is responsible for high luminous efficacy, excellent color quality and a white color of the lamps. Characteristic rare-earth iodides for CMH lamps may include one or more of DyI3, HoI3, TmI3, LaI3, CeI3, PrI3, and NdI3.
Conventional HID lamps with ceramic arc tubes (such as High Pressure Sodium (HPS) and Ceramic Metal Halide (CMH) lamps) have arc tube designs of a “box-shaped” (cylindrical) geometry. This geometric limitation is essentially due to restrictions of early ceramic arc tube manufacturing technologies such as, for example, extrusion of the center body tube component and pressing of flat disk-shaped arc tube end parts (also referred to as “plugs”). As a consequence of the cylindrical geometry, conventional CMH lamps do not operate at a quasi-uniform temperature distribution across the entire center body portion of the arc tube. In particular, some regions of the discharge chamber of a conventional CMH arc tube may be cooler than others even during high-temperature steady-state operating conditions, and these relatively cooler regions form multiple local “cold spot” locations. Cylindrically shaped CMH arc tube designs exhibit cold corners which act as local cold spots, especially at the interface portion of the plug surface that closes off the cylindrical discharge chamber and the surface of the cylindrical center body tube. The vaporized metal halide salt within the discharge chambers of CMH lamps (such as sodium iodide vapor) may be present in a saturated vapor phase, wherein the vapor and liquid phases of the molten metal halide salts are in thermal equilibrium and are both present simultaneously. The equilibrium vapor pressure over the liquid phase is controlled by the temperature of the liquid phase which usually equals the temperature of the “coldest spot” on the internal surface of the wall of the discharge chamber, since this physical point and its surrounding area is the place where the vapor first condenses. However, once condensed, the flow of this liquid condensate is controlled by gravity so that it flows in a downward direction. If the condensed dose flows to a locally hotter location on the internal surface of the discharge chamber then it re-evaporates quickly, and such quick evaporation of the dose droplets results in spikes in temporal vapor dose density of the discharge plasma. Such spikes in vapor dose density in turn generate voltage spikes in lamp electrical characteristics, which also may result in spikes of light intensity and in correlated sudden color changes of emitted light from the lamp. Such spikes in light intensity and the associated sudden color changes are undesirable and are disturbing in high quality lighting environments such as, for example, in retail location lighting.
In designs where the two opposing electrodes of the CMH arc tube are moved further, away from each other, the light emitting electric arc discharge between them becomes a line emitter, and the surface of quasi-equal irradiation turns out to be an ellipsoid, which is still a member of the “spheroid-like” discharge chamber geometries. Such a concept has been used as the basis for shaping QMH discharge chambers in the past, and this same concept is currently being used to design state-of-the-art shaped CMH discharge chambers.
However, the heat radiation from the hot electrode tips reaching the internal surface of a CMH discharge chamber must also be taken into account. This additional irradiation from the electrodes on the arc tube wall can locally increase temperatures of some points on the end portions of the discharge chamber, which end portions are the interface areas where the central body portion of the arc tube meets the elongated tubular sealing portions (also referred to as “legs”) of a CMH arc tube. Thus, when a CMH lamp is operating in a vertical orientation, localized heat radiation from the electrode can re-evaporate the liquid metal halide dose that is flowing down along the inside surface of the discharge chamber wall due to gravity. If the CMH arc tube is of a “ball-shape” design that consists of two hemispheres and which may also additionally include a cylindrical section at the arc tube center) vertical operation of the lamp is especially problematic because potential local overheating and re-evaporation of the liquid dose droplets may easily occur at the bottom body-leg interface section (the “body-leg transition portion”) of such a CMH arc tube. This may occur because the hemispherical end portions of a ball-shaped arc tube design are not perfectly fitted to a heat radiation field of a line emitter, and cannot accommodate the additional localized heat flux from the electrodes. This phenomenon of electrical, light and color instabilities due to liquid dose movement and re-evaporation results in temporal color instability and increased color variability of a CMH lamp, which is often referred to as “dose instability”.
A proposed solution to the problem of dose instability involves preventing the liquid metal halide dose from flowing down to locally hotter surfaces by providing a ring-like mechanical barrier or “nub” on the inside surface of the arc chamber to surround the electrode assembly (at the body-leg transition portion). If the vertical dimension (height) of such a nub is high enough to stop or block the vertical flow of the liquid dose from reaching the overheated point on the internal surface of the arc tube close to the electrode tips, dose instability can be significantly reduced or completely eliminated. However, such a nub creates sharp points on the ceramic arc tube body, and the nub may become the hottest part of the entire end portion of the ceramic arc tube body due to electrode heating. As a consequence, the nub and surrounding area may be exposed to the highest mechanical stresses and may be susceptible to forming cracks in the ceramic material. These cracks can then propagate to lower stress regions and may cause the arc tube to fully crack or even rupture during operation. In addition, some metal halide dose mixtures may operate to quickly erode the nub to such an extent that the nub cannot fulfill its dose stabilization function over the entire life of the lamp.
Another proposed solution for the problem of dose instability involves increasing the emissivity of the arc tube material at the locally overheated body-leg transition portion to promote more efficient cooling of the arc tube wall in this area. However, such a solution can alter or reduce the material strength of the wall, and especially at the most critical area where thermally induced stresses are high enough to crack the arc tube, which can again result in reduced lamp life. Furthermore, in practice controlling emissivity of the ceramic material locally is difficult, and excessive and uncontrolled cooling of the body-leg interface portion (which is also a cold spot location) of such CMH arc tubes may reduce equilibrium vapor pressures of metal halide salts too much, which can result in degraded lamp performance.
Yet another proposed solution for dose instability involves using an ellipsoidal-shaped transition zone between the arc tube center body portion and the body-leg interface portion. However, using an ellipsoidal-shaped transition zone limits geometrical flexibility of the shape both of the body-leg transition zone as well as that of the overall arc tube, and adds unnecessary complexity to the tooling of the ceramic arc tube forming process.